Method and system for indirect determination of local irradiance in an optical system

ABSTRACT

The invention concerns a method for the indirect determination of local irradiance in an optical system; wherein the optical system comprises optical elements between which an illuminated beam path is formed and a measurement object which absorbs the radiation in the beam path at least partially is positioned in a partial region of the beam path selected for the locally-resolved determination of the irradiance and the temperature distribution of at least one part of the measurement object is determined by means of a temperature detector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/343,471, filed Jan. 31, 2006, which claims priority to German PatentApplication 10 2005 004 460.3, filed on Feb. 1, 2005. The fulldisclosure of these earlier applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a method for the indirect determination of localirradiance in an optical system, in particular for a partial region ofan optical system, such as the beam profile at a selected site of thebeam path or of the irradiance on a component of the optical system,wherein the method is applied, in particular, to an optical system withan EUV light source as an illumination source.

2. Description of the Related Art

Optical systems with EUV illumination sources are developed, inparticular, for EUV lithography systems, for the purpose of obtaining,by use of wavelengths of ≦193 nm, pattern widths for electroniccomponents in the submicron range. For this lithography technique withsoft x-rays, so-called EUV lithography is preferred, the wavelengthrange being from λ=11 nm to 14 nm and, in particular, λ=13.5 nm, wherebythe use of numeric apertures in the range of 0.2-0.3 is discussed. Forexample, synchrotron sources or plasma sources can be used asillumination sources for this wavelength region.

The image quality in EUV lithography is determined, on the one hand, bythe projection objective, and, on the other hand, by the illuminationsystem. The illumination system will provide an illumination that is asuniform as possible of the field plane, in which the pattern-bearingmask, the so-called reticle, is disposed. The projection objectiveimages the field plane in an image plane, the so-called wafer plane, inwhich a light-sensitive object is disposed. Projection exposure systemsfor EUV lithography are designed with reflective optical elements. Theshape of the field of an EUV projection exposure system is typicallythat of an annular field with a high aspect ratio of 2 mm (length of thescanning slit)×22-26 mm (width of the scanning slit). The projectionsystems are usually operated in scanning mode, whereby the reticle willbe moved in the field plane and the light-sensitive object, typically awafer with a suitable photoresist, will be moved synchronously in theimage plane, relative to one another. With respect to EUV projectionexposure systems, reference is made to the following publications:

-   W. Ulrich, S. Beiersdorfer, H.J. Mann, “Trends in Optical Design of    Projection Lenses for UV-Lithography and EUV-Lithography” in    Soft-X-Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen    (Editors), Proceedings of SPIE, Vol. 4146 (2000), pages 13-24 and-   M. Antoni, W. Singer, J. Schultz, J. Wangler, I. Escudero-Sanz, B.    Kruizing a, “Illumination Optics Design for EUV-Lithography” in Soft    X-Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (Editors),    Proceedings of SPIE, Vol. 4146 (2000), pages 25-34.

A problem that occurs particularly for optical systems in the EUV rangeis that beam profile measurements for obtaining and also for monitoringthe state of adjustment of the optical components of the optical systemare difficult to conduct due to the short wavelengths. Information ofthe beam profile is also of advantage for evaluating the performancecapacity of optical components.

This same problem arises also in determining the irradiance on theoptical components used in the illumination system. This determinationis of particular importance for EUV illumination systems, since herereflective optical components are used exclusively, which can beconstructed as grazing-incidence or normal-incidence systems.Characteristic of such EUV optics is their limited reflectivity, whichdeteriorates in operation, due to deposits or possible degradationdefects on the mirror surfaces. This problem again makes it necessary toregularly examine the local irradiance, at least in partial regions oron selected subcomponents of such an illumination system, whereby, inparticular, in addition to the characteristic of spatial irradiance, aquantitative determination or a sufficiently accurate estimation of theirradiance is also required.

The methods that have become known from the prior art for monitoringoptical systems in the EUV region concern EUV light sources almostexclusively. Thus, in US 2003/0146391, a detector is proposed formonitoring the irradiated light power of an EUV plasma source, which isfound in a detection beam path separate from the illumination beam path.Here, the étendue value of the detection beam path is adapted to anyillumination beam path in order to simulate this as precisely aspossible. It is a disadvantage in US 2003/0146391, however, that themeasurement of the irradiated light power in a separate beam path isinsufficient by itself to assure that an error with an effect on theimaging in the image plane does not occur, namely an inhomogeneouslyilluminated field, a telecentric error, or a dose error.

The determination of secondary electrons, which come from an absorbingmask for proximity exposure with x-ray light, has become known from JP63-072,116. A locally-resolved measurement for the determination of doseerrors or a contamination of optical components, however, is notpossible.

A method for monitoring the degree of fouling of a mirror forsynchrotron radiation has become known from JP 05-288,696. Here, thephotocurrent integrated over the mirror surface is determined, but thereis no information on the local distribution of irradiance.

Measurements of the beam profile, which are the subject of the presentapplication, are usually conducted with semiconductor detectors, inparticular, in the visible or infrared regions of the spectrum. Atypical field of application is the measurement of laser beam profilesor the determination of illumination characteristics of illuminationsystems. Usual here is the use of silicon, germanium or gallium arsenidedetectors, which are comprised of a combination of a linear detectorwith a pin diaphragm and a precise positioning system, which can be usedfor the scanning measurement of a beam profile. Alternatively, areadetectors, such as CCD sensors or CMOS sensors, which are usuallyintegrated in a camera system, are used for this purpose. The advantageof area detectors when compared with linear detectors, above all, is thesavings of time when conducting a measurement.

With respect to beam and illumination diagnostics for illuminationsystems which use short wavelengths, in particular in the EUV region, afluorescence converter can be utilized, with which radiation in awavelength region of 10 nm up to approximately 350 nm can besuccessfully converted into the visible wavelength region, so that astandard camera with a Si-CCD detector can be used for taking images. Itis a disadvantage with the use of fluorescence converters, however, thatthey are suitable only to a limited extent for beam profile measurementsfor illumination systems with a high illumination intensity. Thisapplies, in particular, to the EUV region in which the absorbedradiation rapidly leads to an overheating and to the degradation of thefluorescence converter or to a change of the conversion efficiency andthus leads to a falsification of the measurement. This limitationessentially applies also to the DUV and the VUV wavelength regions, inaddition to the EUV wavelength region. Thus, EUV photodiodes in row ormatrix arrangement, which measure the photocurrent or photoelectrons ina locally-resolved manner, are preferred for the EUV region.

Additional requirements for a method or a measurement system fordetermining the local irradiance in an optical system for the wavelengthregion of ≦193 nm and, in particular, in the EUV region, result from therequirements relative to constructability and vacuum conditions. Theknown detector systems often can be integrated into the illuminationbeam path and encapsulated by vacuum technology only at increasedexpense. Furthermore the problem occurs that such detector systemsnecessarily interrupt the beam path during the measurement and,therefore, further measures are necessary for positioning of allmeasurement components used for the measurement of the beam profile orthe illumination characteristic. Continuous measurements or routineinspections, which can be conducted without interfering with the stateof adjustment, are thus possible only with difficulty with the knowndetection systems.

Another disadvantage with the use of the known measurement systems fordetermining beam profiles or the local irradiance on an opticalcomponent results from the fact that detectors are adapted to thewavelength region that is used in each case, for example, by means of acoating layer using a film filter, so that common detection systems arenot suitable for broadband spectra—IR, VIS, UV, DUV, VUV and EUV. Auniversal measurement method or measurement system that can be used overa broad wavelength region is of advantage in itself. For applications inEUV lithography systems, in particular, an adjustment can then becarried out with adjustment illumination outside the used wavelengthsfor EUV, without the need for changing the measurement system. The sameset of problems is also to be encountered in optical systems for the DUVand VUV wavelength regions.

SUMMARY OF THE INVENTION

The object of the invention is to overcome the above-describeddisadvantages of the prior art and to provide a method and a measurementsystem associated with it, with which it is possible in a simple mannerto determine the beam profile in an optical beam path or the irradianceon a component of an optical system, which is provided in particular forthe short wavelength region of ≦193 nm and also, in particular, for theEUV region. This determination should be sufficiently accurate so thatthe spatial intensity distribution of the radiation in the beam path, inparticular the beam profile or the irradiance on an optical componentcan be detected and, in particular, a quantitative determination of theirradiance results. In addition, the method should be universallyapplicable, i.e., for a broad wavelength region and, in particular, forthe spectral regions of IR, VIS, UV, DUV, VUV and EUV. Further, themeasurement method and the measurement system belonging thereto will besuitable for routine measurements in the optical system to be monitored,i.e., measurements can be conducted without a change in the state ofadjustment and with only small expense for the alignment of themeasurement system. In addition, the measurement system will be robust.

For the method according to the invention, the inventors have recognizedthat with a locally resolved measurement of the heating up of an opticalcomponent of an optical system due to the illumination radiation thatoccurs, which is absorbed at least partially, an indirect determinationof irradiance is possible. For this purpose, by means of a temperaturedetector, for example, a camera that is sensitive in the infraredwavelength region or an arrangement of temperature sensors in thermalcontact with the measurement object, the spatial temperaturedistribution on the optical component of the optical system selected forinvestigation is determined. These measurements can be conducted eitherduring the warming up or after a stationary temperature distribution hasbeen adjusted.

Corresponding to a variant according to the invention, instead of anoptical component of the optical system, an object absorbing theillumination radiation in the beam path of the optical system isintroduced for measurement at the site of the illumination beam path tobe investigated and the temperature which is adjusted on its surface isdetermined. After the measurement, the object which has been introducedis removed again. In the present invention, the term measurement objectis thus used both for an optical component of the optical systemselected for the temperature measurement as well as also for a specialobject introduced into the beam path only for measurement purposes.

Suitable detectors for determining the temperature of the measurementobject are, for example, measurement systems operating in a contact-freemanner, in particular CCD camera systems sensitive in the infrared or,alternatively, linear detectors for the infrared that operate byscanning. These detectors are thus of advantage, since they candetermine the surface temperature of the selected optical component orof the measurement object without blocking the beam path of the opticalsystem to be investigated.

Alternatively, detectors which are found in direct thermal contact withthe measurement object can also be utilized for temperaturedetermination. It is therefore possible, for example, to introducethermocouples or thermoresistors as a detector matrix on the back of areflective optical component or on facet mirrors, onto the mirrorsurface between the individual facets. Other detector arrangements areconceivable, which are suitable for the purpose of sufficientlyprecisely determining the spatial temperature distribution on theirradiated surface.

A measurement object can be selected correspondingly so that one canproceed essentially from a known absorption at the surface for theradiation in the optical system. A measurement object is thenparticularly well suitable for conducting the method according to theinvention, if its absorption behavior approximates as much as possiblethat of a black box. In the case of an ideal black box, the surfacecompletely absorbs the incident illumination radiation and emitselectromagnetic waves corresponding to the surface temperature, i.e., itemits with the maximum spectral emission capacity according to Planck'sLaw. The irradiance of the incident radiation can be derivedparticularly precisely from the temperature determination for such apreferred measurement object.

The starting point for the relationship between the heating of thesurface of a measurement object or an optical component is the generalequation for heat conduction:

${\overset{.}{T} = {{\frac{\lambda}{\rho \; c}\Delta \; T} + \overset{.}{Q}}},$

which describes the change in the temperature T over time. Here, {dotover (T)} is the time derivative of the temperature, λ is the heatconductivity, c is the specific heat capacity, ρ is the density and {dotover (Q)} is the source density for the internal heat input, in thiscase caused by the illumination radiation absorbed near the surface.

For the method according to the invention, it is preferably assumed thatthe illumination radiation is absorbed only by the layer directly at thesurface. It is thus true for the surface layer that the source term {dotover (Q)} in the heat conduction equation is approximately proportionalto the local irradiance. Within the scope of the knowledge of the personskilled in the art, it is possible, however, to proceed from a modelassumption for the depth of penetration of the electromagneticradiation.

In a first approximation, the change in the surface temperature overtime thus can be set up as a linear dependence relative to the sourceterm Q and thus as proportional to the local irradiance. Correspondingto this consideration, the diffusion term

$\frac{\lambda}{\rho \; c}\Delta \; T$

of the heat conduction equation can be disregarded initially. This ispossible if one can start with a small temperature conductance value(λ/ρc). In addition to the possibility of influencing the magnitude ofthe temperature conductance value by an appropriate selection ofmaterial, the diffusion term is then small, if the spatial temperaturedifferences are still sufficiently negligible. It is thus of advantageto carry out the temperature determination directly after adiscontinuity in the irradiance and to compare it with a temperaturedistribution assumed to be stationary prior to the discontinuity. Itfollows from this in turn that a particularly sensitive detector shouldbe used preferably for the determination of such a temperature changevs. time, which is called the “initial” in the following.

For an optical component active in the optical system, a discontinuityin the irradiance can be achieved, for example, by turning on theillumination or correspondingly by operating a diaphragm. For ameasurement object that does not permanently remain in the beam path,the described discontinuity in the irradiance can be effected by asufficiently rapid introduction into the beam path. Spatial differencesresult for the initial heating up of the irradiated measurement surface,as a function of the beam profile of the radiation impinging on themeasurement object.

It is also of advantage with respect to the local resolution of aselected measurement object, if the lateral temperature conductance isas small as possible. The use of a segmented structure is advantageous;for example, this can be of a raster shape, in which the individualsegments are thermally separated from one another as much as possible.According to an advantageous embodiment, the temperature of each ofthese individual segments under irradiation is then determined. Asimilar advantageous measurement situation results with the use of afacet mirror as an optical component of an optical system as themeasurement object, since, when forming the facets, it is possible toprovide for a sufficient thermal separation between the individualfacets. When a mirror with a continuous surface is used, a lateralthermal decoupling can be provided by the appropriate configuration ofthe mirror unit with respect to selection of material and geometry.Thus, e.g., it is possible to employ recesses or an arrangement oflayers of material with a high resistance to heat transfer in thetransition region between individual segments of the mirror unit.

For the initial temperature determination, a sequence of temperaturemeasurements is preferably to be conducted at known time points and thusa temperature course is determined, from which the initial change intemperature vs. time resulting upon a discontinuity in the irradiance attime point T₀ can be determined particularly precisely as a tangent totime point T₀ of the temperature vs. time curve.

In correspondence with one variant of the method according to theinvention, it is conceivable not only to determine the initialtemperature change vs. time at the irradiated surface of the measurementobject or of the investigated optical component and to disregard thediffusion term of the heat conduction equation, but to estimate theenergy input and thus the irradiance from the temperature curve vs. timeand space during heating up. This presumes that the geometry of theobject as well as the spatial course of the temperature conductance andof the heat outflow at the boundaries of the object are known or can besufficiently precisely estimated. In general, it will not be possible toprovide consistent solutions for this, so that numerical methods, forexample, the method of finite elements must be employed. In addition, itis of advantage in this case, if not only the temperature of theirradiated surface is measured, but also if the temperature curve of theentire body of the measurement object or of the optical component can beplotted by means of sensors that are very closely distributed in space.This variant of the method according to the invention is morecomplicated both in terms of measurement technology as well as in termsof modeling, but it can be of advantage, if a discontinuity of theirradiance can be conducted without anything further, since furthermeasures would mean an interruption of the operation of the illuminationsystem. In particular, optical components can be continuously monitoredduring operation, when the change in temperature vs. time is notdetermined, but when the irradiance can be derived from measurements ofthe stationary temperature gradients.

For the case when an optical component of the optical system is used asthe measurement object, and, in particular, if stationary temperaturegradients are measured, information on the deformation of the opticalcomponent due to thermal load can be estimated from the temperaturedistribution. It is then possible to construct the optical component asadaptive optics and to readjust the geometric shape of the functionalsurface of the optical component based on the temperature data. Thechange in the optical imaging that results and thus also the irradianceon the optical component of the optical system can in turn be determinedby the method according to the invention, so that a regulation of theadaptive optics utilized is made possible.

If, instead of a measurement object, an optical component of the opticalsystem is used directly for the indirect determination of the localirradiance, then there may occur, in particular, if a flat-surfacemirror is involved, the additional difficulty that the reflectivity isnot constant over the optically active surface. Consequently, it isnecessary to determine sufficiently accurately the absorbed portion ofthe illumination radiation starting from model assumptions or frommeasurements, so that it is possible to draw conclusions on the localirradiance from the temperature measurements.

In a reverse approach, on the other hand, it is possible to findinformation on the local reflectivity from the temperature measurementsand thus from the determination of the absorbed heat power. The localirradiance is presumed to be known for this case. This can beaccomplished, for example, by conducting the method according to theinvention on another optical component or an additional measurementobject or by directly monitoring the illumination source and thusestimating the irradiance at the site of the optical component to beinvestigated. In correspondence with this variant of the invention,information on the local reflectivity results from the temperaturemeasurements, which can be used, in particular, for monitoring opticalcomponents, particularly of reflective and consequently easily fouledsurfaces in the optical system. Such measurements can be conductedroutinely after specified operating times in order to obtain informationon the degree of fouling and possibly a degradation of the opticallyactive surfaces in an optical system.

If one of the optical components of the optical system is used directlyfor the method according to the invention for the temperaturemeasurement, then continuous measurements are possible withoutinterrupting the function of the optical system. With the alternativeuse of a separate measurement object, only one part of the measurementmeans used for the measurements of the beam profile is introduced in thebeam path for conducting the measurement, while, for example, thetemperature detector itself can be positioned outside the opticalsystem. This minimizes the apparatus expense, in particular in opticalsystems in the EUV region, which are kept under vacuum conditions, sincethe temperature detector is not encapsulated by vacuum technology.Further, the measurement method is robust and universally applicable,since the detection system itself need not be adapted to the radiationused in the optical system and only standard temperature detectors areneeded.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described below by way of examples based on thefigures.

FIG. 1 a: Optical system with an optical beam path and a heat imagecamera;

FIG. 1 b: Optical system with an optical beam path and a heat imagecamera for recording the temperature distribution on a measurementobject;

FIG. 2: Optical system with an optical beam path and a heat image camerafor recording the temperature distribution on a measurement object whichcomprises a spectral filter;

FIG. 3: Optical system with an optical beam path and a heat image camerafor recording the temperature distribution on a measurement object, witha spectral filter separated from the measurement object, which isdisposed in the beam path upstream of the measurement object;

FIG. 4: Construction of the measurement object with segmented structurefor limiting the lateral heat conductivity;

FIG. 5 a: Arrangement of thermoresistors or thermocouples on an opticalmirror component;

FIG. 5 b: Arrangement of thermoresistors or thermocouples on facets of afacet mirror;

FIG. 5 c: Arrangement of thermoresistors or thermocouples in thevicinity of facets of a facet mirror according to FIG. 5 b withadjusting elements for the facets for the formation of adaptive optics;

FIG. 6: Schematic representation of a facet mirror;

FIG. 7: Sketch of an EUV lithography system comprising an illuminationsystem and a projection system.

DESCRIPTION OF THE INVENTION

FIG. 1 a shows in a schematically simplified manner an optical system,which comprises a light source 1, from which is emitted an optical beampath 2. The optical beam path 2 is influenced in the optical system byoptical components. Optical components with the reference numbers 10.1and 10.2 for this purpose are shown schematically in FIG. 1 a. Theseoptical components can be refractive or reflective, but only reflectiveoptics can be used for an EUV system. Further, a heat camera 50, whichis directed onto a plane of intersection 52 in beam path 2, is shown inFIG. 1 a.

In accordance with the method according to the invention, according tothe presentation sketched in FIG. 1 b, by means of the temperaturedetector 50, in the present case, a heat camera, the surface temperatureis determined for a measurement object 54, which has been introduced,for purposes of measurement, into the optical beam path 2 in the regionof the plane of intersection 52 in this example of embodiment. In thisway, the heating up process is measured, wherein it is preferred todetermine the initial temperature change vs. time on the surface of themeasurement object 54. The change in surface temperature vs. time isunderstood as occurring directly after a discontinuity in the irradianceon the surface of the measurement object. This can be effected, forexample, in such a way that the temperature on the surface of themeasurement object is determined prior to its introduction into theoptical beam path 2 and then directly after this introduction.

For optical systems which operate in a broadband wavelength region, itmay be of advantage to spectrally filter the radiation impinging on themeasurement object. Particularly for EUV systems, it is desired thatonly the wavelength region used for the useful application impinges onthe measurement object in order to determine the beam profilecharacteristic relevant to the optical system. A spectral filter 56 canbe assigned directly to the measurement object 54, which is shownschematically in FIG. 2. Thus it is possible to hold the components ofthe measurement object 54 and the spectral filter that are heated by theincident radiation in a common holder or to use a film filter directlydeposited on the measurement object. The film filter can be formed hereso that it selectively absorbs the investigated wavelength region usedand to the greatest extent possible reflects the wavelength regions thatare to be filtered out. Alternatively, it is also possible to create thespectral filter 56 separately from the measurement object. This filtercan then be positioned at a place in the beam path which extends out infront of the measurement object. A corresponding configuration is shownin FIG. 3, which presents an arrangement of a spectral filter 56positioned in the beam path. A possible configuration of such a spectralfilter is a raster spectral filter consisting of a diffraction gratingin combination with a diaphragm arrangement.

If a measurement object is introduced into the beam path and issubjected to the irradiation, then in accordance with a preferredmeasurement variant, the temperature measurement begins immediately bymeans of the heat detector. The body will be heated up successively,i.e., with increasing heating, the temperature measurement signal isalso easier to determine due to the increasing signal-to-noise ratio,but an obliteration of the temperature differences brought about by thespatially different irradiation is, of course, also encountered due todiffusion effects. Since diffusion terms in the heat conduction equationcomplicate the assignment of heating to the actual incident irradiation,

a material which has a low heat conductivity is used particularly as themeasurement object. In the present application, a low heat conductivityλ will be understood to mean a heat conductivity that lies in the rangeof 0<λ≦2 ^(W)/_(mK). A material with a low heat conductivity is glassceramics, for example Zerodur® of the Schott Glas company, which has aheat conductivity of λ≈1.46 ^(W)/_(mK) at a temperature of 20° C.

Other materials with low heat conductivities are natural quartz orsynthetic quartz with a heat conductivity of 1.38 ^(W)/_(mK) at 20° C.as well as different types of vitreous materials, for example, windowglass, which has heat conductivites, for example, in the range of 0.8^(W)/_(mK)≦λ≦1.15 ^(W)/_(mK). Corresponding to an advantageousconfiguration of the measurement object, the particularly disruptivelateral heat conductivity is reached by a segmented formation of thesurface of the measurement object. A possible configuration as a rasterstructure is sketched in FIG. 4. At the left is shown a view from thetop, and at the right is a section along line A-A. Here, the individualsegments with sufficiently good conductivity are surrounded by amaterial of lower heat conductivity,

whereby this layer of lower heat conductivity should be disposedlaterally to the direction of irradiation. It is also possible to effecta drawing off of temperature by means of a cooling that typically actson the back side of the irradiated surface of the measurement object. Byintentionally building up temperature gradients essentially in thedirection of the irradiation causing the input of heat, the undesiredlateral obliterating of the temperature curve is reduced.

FIGS. 5 a and 5 b show the variants of the method according to theinvention, according to which the temperature measurement is not carriedout on a separate measurement object, but rather by the determination ofthe surface temperature of an optical component acting in the opticalsystem. To this end, FIG. 5 a shows the arrangement of thermocouples orthermoresistors 66.1, 66.2, 66.3 on the back of a flatly formed mirror64. It is particularly preferred here to position the thermoresistors orthermocouples 66.1, 66.2, 66.3 used as temperature detectors as close aspossible to the irradiated surface.

A variant in the design of the optical component is shown in FIG. 5 b.Shown here are individual facets 500.1, 500.2, 500.3 and 500.4 of afacet mirror, wherein the heat measurement by means of the temperaturedetectors, which are thermoresistors and thermocouples 66.1, to 66.3 inthe case shown, is again performed individually on one of theseindividual facets 500.1-500.4. A temperature measurement is preferablyconducted at each of these individual facets. A frontal view of such afacet mirror is depicted in FIG. 6. Here again, individual facets arelabeled 500.1-500.10. Facet mirrors are thus preferred for conductingthe method according to the invention, since, due to the surfaceconfiguration, it is well possible to effect a sufficient lateralthermal decoupling of individual segments of the mirror surface of theoptical component. In addition, it is conceivable to accommodate thethermocouples in the intermediate regions between the individual facets,so that these can be disposed as close as possible to the irradiatedsurface. The diffusion terms that are disruptive for the indirectdetermination of the local irradiance play a smaller role in the heatconduction equation due to these measures.

The indirect determination of the irradiance on an optical componentaccording to the invention by means of temperature measurement canadvantageously be combined with adaptive optics. In FIG. 5 c is depicteda corresponding arrangement, which in addition to the components shownin FIG. 5 b, has adjusting elements 68.1, 68.2, 68.3, with which thefunctional surfaces, in this case, the individual facets of the facetmirror, are readjusted in their position and alignment as a function ofthe temperature data.

A projection exposure system to which the adjustment process accordingto the invention can be applied is depicted in FIG. 7. Proceeding in thelight path from a light source 1 to an illuminated plane, which iscalled the field plane 13, are shown the optical components of anillumination system and also the projection objective 126.

The following are shown individually in FIG. 7: A reticle or a mask 11is positioned in the field plane 13 of a projection exposure system, inwhich preferably an annular field is formed, and is imaged by means of areduction optics 126 on its imaging plane 130, in which typically awafer 106 provided with a light-sensitive material is found. FIG. 7shows for this purpose, as an example, a projection objective consistingof six individual mirrors 128.1 to 128.6, which is derived, for example,from U.S. Pat. No. 6,600,552, which is incorporated to the full extentin the present application. Also depicted in a telecentric illuminationof image plane 130, which is found in the ideal case, is the chief rayof a beam bundle, which proceeds out from a field point of field plane13, and perpendicularly intersects the image plane 130. In addition, theprojection objective 126 has an entrance pupil which in generalcoincides with the exit pupil of the illumination system.

FIG. 7 also shows the typical structure of an EUV illumination system,which is formed as a double-facetted illumination system according toU.S. Pat. No. 6,198,793 B1, whereby the content of this document isincorporated to the full extent in the present Application. Such asystem comprises a first optical element with first raster elements 3,which is also designated as field facet mirror 3. A second opticalelement with second raster elements 5, which is usually named a pupilfacet mirror 5, then follows in the beam path.

Field facet mirror 3 and pupil facet mirror 5 serve for illumination ofa field in the field plane 13 as well as the shaping of the illuminationin the exit pupil of the illumination system. The action of each fieldraster is such that it forms an image of light source 1, wherein aplurality of so-called secondary light sources is formed by theplurality of field facets. The secondary light sources are formed in ornear the plane in which the pupil facet mirror 5 is disposed. Thus, asshown in FIG. 7, if the secondary light sources come to lie in theregion of the pupil facet mirror 5, the field facets themselves can havean optical effect, for example, a collecting optical effect. Thesesecondary light sources are imaged by the downstream optical elements astertiary light sources in the exit pupil of the illumination system.

In addition, each field raster is imaged in the field plane 13 by thefacets of the pupil facet mirror 5 and the downstream optical elementsof the second optical component 7, which, in the example of FIG. 7,consists of the following three optical elements: a first reflectiveoptical element 19, a second reflective optical element 21 and agrazing-incidence mirror 23. The images of the field facets that aresuperimposed there serve for the illumination of a mask 11 in the fieldplane 13, whereby, typically, starting from rectangular or arc-shapedfield facets, an illumination in the form of an annular field segmentarises in the field plane 13. In general, the microlithography system isformed as a scanning system, so that the mask 11 in the field plane 13and a wafer 106 in the image plane 130 are moved synchronously in orderto effect an illumination or an exposure.

The method according to the invention for preliminary adjustment can beapplied with advantage also to optical systems outside the EUVwavelength region. In addition, an application to a plurality of opticalsystems is possible—to illumination or projection systems, for example.An example of the latter are objectives which are provided for the VUVor the DUV regions.

Within the scope of the invention, a projection exposure system formicrolithography is also disclosed, in which, in its illumination orprojection system, at least one temperature detector is integrated inorder to carry out the method according to the invention. In addition,the invention also comprises a measurement stand for adjusting or fordimensioning optical systems, this stand possessing at least onetemperature detector for conducting the method for the indirectdetermination of the local irradiance.

LIST OF REFERENCE NUMBERS

-   1 Light source-   2 Optical beam path-   3 First optical element with first raster elements (field facet    mirror)-   5 Second optical element with second raster elements (pupil facet    mirror)-   7 Second optical component-   9 First collector unit-   10.1 Optical component-   10.2 Optical component-   11 Pattern-bearing mask-   13 Field plane-   19 First reflective optical element-   21 Second reflective optical element-   23 Grazing-incidence mirror-   50 Temperature detector-   52 Plane of intersection in the optical beam path-   54 Measurement object-   56 Spectral filter-   60 Regions of good heat conductivity on the measurement object-   62 Regions of poor heat conductivity on the measurement object-   64 Flat-shaped mirror of an optical component-   66.1, 66.2, 66.3 Thermoresistors or thermocouples-   68.1, 68.2, 68.3 Adjusting elements for adaptive optics-   102 Second collector unit-   106 Wafer provided with a light-sensitive material-   126 Projection objective-   128.1, 128.2, 128.3 Mirrors of the projection objective-   128.4, 128.5, 128.6-   130 Image plane-   200 Raster element-   202 Diaphragm plane-   204 Diaphragm in the first collector unit-   400 Normal-incidence concave mirror-   410 Cleaning chamber-   500.1, 500.2, 500.3 Individual facets-   500.4, 500.5, 500.6-   500.7, 500.8, 500.9-   500.10-   Z Intermediate image of the light source

1. A method, comprising: providing an optical system that comprises:optical elements configured to form a beam path for radiation passingthrough the optical system; and a measurement object at least partiallypositioned in the beam path; passing the radiation through the opticalsystem; and using a detector to measure a parameter selected from thegroup consisting of a temperature distribution of at least part of themeasurement object, an irradiance of the radiation on the measurementobject, and a spatial intensity distribution of the radiation on themeasurement object, wherein the measurement object is a mirror.
 2. Themethod of claim 1, further comprising adjusting a shape of the mirrorbased on the measured parameter.
 3. The method of claim 2, wherein themirror comprises a plurality of facets.
 4. The method of claim 3,wherein adjusting the shape of the mirror based on the measuredparameter comprises adjusting a position of the plurality of facetsbased on the measured parameter.
 5. The method of claim 4, whereinadjusting the shape of the mirror based on the measured parameterfurther comprises adjusting an alignment of the plurality of facetsbased on the measured parameter.
 6. The method of claim 3, whereinadjusting the shape of the mirror based on the measured parameterfurther comprises adjusting an alignment of the plurality of facetsbased on the measured parameter.
 7. The method of claim 3, furthercomprising individually adjusting an alignment of the plurality offacets based on the measured parameter.
 8. The method of claim 7,further comprising individually adjusting a position of the plurality offacets based on the measured parameter.
 9. The method of claim 3,further comprising individually adjusting a position of the plurality offacets based on the measured parameter.
 10. The method of claim 1,wherein the mirror is adaptive in response to the measured parameter.11. The method of claim 1, further comprising estimating deformation ofthe mirror based on the measured parameter.
 12. The method of claim 1,further comprising adjusting a functional surface of the mirror based onthe measured parameter.
 13. The method of claim 12, wherein the mirrorcomprises a plurality of facets.
 14. The method of claim 1, whereinpassing radiation through the optical system images an object field ofan object plane of the optical system to an image field of an imageplane of the optical system.
 15. The method of claim 14, furthercomprising changing the imaging by the optical system by adjusting themirror based on the measured parameter.
 16. The method of claim 14,wherein the optical system is a lithography projection objective. 17.The method of claim 1, wherein the optical system is a lithographyprojection objective.
 18. The method of claim 1, wherein the opticalsystem is a projection objective for EUV lithography.
 19. The method ofclaim 18, further comprising adjusting a shape of the mirror based onthe measured parameter.
 20. The method of claim 1, wherein the measuredparameter is the temperature distribution of at least part of themirror.
 21. The method of claim 1, wherein the measured parameter is theirradiance of the radiation on the mirror.
 22. The method of claim 1,wherein the measured parameter is the spatial intensity distribution ofthe radiation on the mirror.
 23. The method of claim 1, furthercomprising altering imaging properties of the optical system based onthe measured parameter.
 24. The method of claim 23, wherein altering theimaging properties of the optical system comprises adjusting a shape ofthe mirror.
 25. The method of claim 24, further comprising, aftermeasuring the parameter, removing the mirror from the beam path.
 26. Amethod, comprising: providing a lithography projection objective,comprising: optical elements configured to form a beam path forradiation passing through the lithography projection objective; and amirror at least partially positioned in the beam path, the mirrorcomprising a plurality of facets; passing the radiation through thelithography projection objective to image an object field in an objectplane of the lithography projection objective to an image field of animage plane of the lithography projection objective; using a detector tomeasure a parameter selected from the group consisting of a temperaturedistribution of at least part of the mirror, an irradiance of theradiation on the mirror, and a spatial intensity distribution of theradiation on the mirror; and adjusting facets of the mirror based on themeasured parameter to change imaging of the lithography projectionobjective.
 27. A system, comprising: an optical system, comprising:optical elements configured to form a beam path for radiation passingthrough the optical system; and a mirror configured at least partiallypositioned in the beam path; and a detector configured to measure aparameter when the radiation passes through the optical system, theparameter being selected from the group consisting of a temperaturedistribution of at least part of the mirror present, an irradiance ofthe radiation on the mirror, and a spatial intensity distribution of theradiation on the mirror.